1. Field of the Invention
The present invention relates to catalyst components for olefin polymerization which can produce polyolefins, and in particular polypropylene having high crystallinity and relationship high melt flow rates (MFR), and consequently, suitable processing properties.
2. Description of the Prior Art
The homopolymers and copolymers of propylene generally have certain properties that are unsatisfactory for specific applications. It therefore becomes necessary to modify certain characteristics during the manufacture of the polypropylene to render the polymer more useful for a certain end results. For example, if the rigidity of the polymer or copolymer of propylene is improved, it is possible to reduce the thickness of the resulting molded product formed from it.
There are numerous polymerization and copolymerization processes and catalyst systems in the prior art from which it is possible to tailor a processing catalyst system to obtain a specific set of properties of a resulting polymer or copolymer. For example, in certain applications, a product with higher melt flow rate is desirable. Such a product has a lower melt viscosity than a product with a lower melt flow rate. Many polymer or copolymer fabrication processes which operate with high shear rates, such as injection molding, oriented film and thermobinded fibers, would benefit from a lower viscosity product by improving through-put rates and reducing energy costs. Generally, olefin polymers obtained by using an active catalyst component of the magnesium (MgCl.sub.2) supported type have a limited melt flow rate range and mechanical properties. As indicated, however, for certain applications, polypropylene polymers which flow readily during melting have improved processing characteristics.
A discovery of more appropriate co-catalysts or electron donors to accompany supported magnesium catalyst components have been of great benefit to improving the efficiency of the catalyst system and the quality control of the polymer product. In such catalyst systems, the cocatalyst activates the catalyst and provides initiation of a polymer chain. The cocatalyst that has historically worked well with magnesium supported catalysts is organo aluminum compounds, most typically triethylaluminum ("TEAL"), or other trialkyl aluminum compounds. Examples of other useful organoaluminum compounds include an alkylaluminum dihalide, a trialkoxyaluminum, a dialkylaluminum halide and a triisobutyl aluminum.
An electron donor compound is used in the polymerization reactor to control the stereoregularity and form of the polymer. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or groups of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor can lead to significant improvement in the properties of the polymer product such as molecular weight distribution and melt flow. Discovery of a specific group of electron donors for magnesium supported catalysts that would provide beneficial results would be highly advantageous.
The present invention is directed to the use of a mixture of silane electron donors. In Mitsui EP 385765A (published Sep. 5, 1990), there is demonstrated a catalyst system composed of a magnesium supported titanium catalyst component in combination with two silane electron donors, a specific donor mixture embodiment of which is directed to dicyclopentyl dimethoxysilane ("DCPMS") and propyltriethoxysilane ("PTES"). In U.S. Pat. No. 5,100,981, disclosed is a catalyst system composed of a magnesium supported titanium catalyst and a mixture of two electron donors, cyclohexylmethyl dimethoxysilane ("CMMS") and phenyltriethoxy silane ("PES").
It has now been surprisingly found that the use of two different organosilicon compounds as electron donors in combination with a magnesium supported catalyst is capable of generating highly isotactic polypropylene polymers having a moderately broad molecular weight distribution as well as relatively high melt flow rates.